1. Field of the Invention
The present invention relates to an ambient light sensor which detects ambient illumination.
2. Description of Related Art
Conventionally, electronic apparatuses that use an ambient light sensor to detect ambient illumination and carry out various kinds of control according to the detected results are in practical use. For example, in the field of photographing apparatuses, typically, a digital still camera and a digital camcorder, exposure control and flash control are carried out according to a detected result from an ambient light sensor. Besides, in the field of liquid crystal display apparatuses, typically, a liquid crystal display television and a liquid crystal display monitor, brightness control of a backlight is carried out according to a detected result from an ambient light sensor.
As an example of the conventional art related to the technology described above, there is JP-A-2002-176192 which is disclosed and proposed by the applicant of the present patent application.
Generally, the conventional ambient light sensor has a structure in which a photoelectric transducing device such as a photodiode and the like is used to generate a light current depending on ambient illumination, and the light current is amplified by a current mirror amplifier to generate a desired output signal (output current).
Because of this, in an ambient light sensor that has the structure described above, to obtain an output signal proportionate to ambient illumination, it is necessary to keep always the mirror ratio of the current mirror amplifier at a constant value.
However, in a case where the current mirror amplifier is composed of a bipolar transistor, the output signal is saturated as the light current increases, and the mirror ratio of the current mirror amplifier deviates from a predetermined target value, accordingly, linearity of the output signal is kept in only a region where the light current is relatively small.
On the other hand, in a case where the current mirror amplifier is composed of a MOS (Metal Oxide Semiconductor) Field Effect Transistor, fluctuation in the output signal becomes large as the light current becomes small, and the mirror ratio of the current mirror amplifier deviates from a predetermined target value, accordingly, linearity of the output signal is kept in only a region where the light current is relatively large.
In the field of ambient light sensors, a technology is developed, in which calculation of a difference between light currents that are separately generated by a first and second photoelectric transducing devices which are different from each other in light receiving characteristic (spectral sensitivity) is carried out, thereby only a desired wavelength is measured.
However, because the ambient light sensor described above is so structured as to always carry out a difference between the light signals according to a constant proportion, depending on certain kinds of light sources whose illuminations are measured, the difference calculation result becomes below zero, and the output signal becomes zero, accordingly, there is a problem that exact measurement of illuminations of the light sources cannot be carried out.